In a semiconductor wafer processing environment, semiconductor wafers must be protected from contaminants and physical agitation. Such wafers are used to manufacture electronic components such as computer memory and microprocessors, and require certain physical characteristics so that electronic circuit elements may be fabricated onto the wafer surface. Manufacturing imperfections, such as those which can arise from dust, dirt, bumping, and jarring, can render a wafer unusable.
Accordingly, such wafers are often stored and transported in a sealable container called a wafer pod, or cassette. Such cassettes have a series of interior ridges on opposing sides to support a batch of wafers horizontally, and a removable door to allow access to the contents. Typically an automated, apparatus, such as a robotic arm or conveyor system, is used to transport these cassettes to minimize human manipulation which can lead to dropping and bumping of a loaded cassette, and further to damage and loss of wafer stock. Such an apparatus transports cassettes between different processing stations during various phases of the wafer manufacturing and treatment process.
At each processing station, the cassette is placed on a support platform which includes an arrangement of pins having beveled tops called a nest, which mate with corresponding beveled receptacles on the bottom of the cassette. The beveled tops allow precise, consistent placement while affording some tolerance of movement when placing the cassette on the pins. A typical prior art pin assembly is shown in FIGS. 1a, 1b. Typically three pin assemblies are used to support a cassette, of which a single traditional pin assembly 10 is shown in FIG. 1a. The beveled edges 12 mate with a corresponding beveled surface 14 on a cassette 16 at a contact area 18, as shown in FIG. 1b.
Removal of the cassette from the nested position on the pin assemblies can involve insertion of a robotic arm, or paddle, under the cassette between the pin assemblies, and lifting upwards. Traditional pin arrangements, however, incorporate cassette sensing pads which reside under the cassette in the area between the pins to sense the presence of a cassette by being displaced downward. Such pads, therefore, interfere with insertion of the paddle underneath the cassette.
A typical cassette receptacle and pin assembly arrangement is shown in FIGS. 2a, 2b, respectively. Two triangular orientations are commonly used. On the bottom side of a cassette 56, a larger, outer pin assembly receptacle orientation 42 is used to support a cassette nested at a processing station, while an inner pin assembly receptacle orientation 44 is used by a robotic paddle arm to transport cassettes between processing stations. The corresponding pin assembly orientations on the processing station 48 are shown in FIG. 2b. The inner set of pins 54 is mounted on a paddle 46, while the outer set 52 corresponds to placement at the processing station 48. Referring to FIGS. 2a, 2b, paddle 46 can effect removal of cassette 56 by being inserted between the outer set of pins 52 beneath the cassette 56, and lifting upward such that inner pins 54 engage inner receptacles 44.
Prior art cassette detection methods using cassette sensing pads 58 are incompatible with the use of the paddle 46 in FIG. 2b. As such pads reside within the paddle exclusion zone 60, they can interfere with the insertion of the paddle 46 between the outer set of pins 52 beneath the cassette 56. Alternative sensor placement is undesirable due to the need to maintain compliance with industry standards, and alternate non-interfering insertion paths of the paddle can complicate design of new systems and may not be suitable for existing paddle systems.